a) Field of the Invention
The invention relates to a method for manufacturing a polymeric secondary battery and, more particularly, to a method for removing plasticizer during a manufacturing process of a polymeric secondary battery.
b) Description of the Related Art
In recent years, the prevalence of portable electronic devices has lead to an increasing demand for better secondary batteries (rechargeable batteries) to use therein. Secondary batteries commonly used are polymeric secondary batteries, which include two types: one type uses a polymer to form an electrode, and the other uses a polymer to form an electrolyte. Since the latter can prevent problems such as leakage, volatilization, and burning of the traditional electrolyte solution, the industry is actively developing secondary batteries of this type
However, traditional solid state polymers have poor conductivity, which causes a deficiency in the electrical characteristic of a second battery that uses solid state polymer to form the electrolyte. A polymeric secondary battery disclosed in U.S. Pat. No. 5,540,741 by Bell Communication Research successfully solves this problem. The disclosed polymeric secondary battery is characterized by using a special polymer material (PVDF-HFP) and plasticizer to make a positive electrode, a negative electrode, and a separator membrane, after which they absorb an electrolyte solution and forms a colloidal polymeric electrolyte having good conductivity.
FIG. 1 is a flow chart of the manufacturing process of the polymeric secondary battery according to the patent. First in step 11, a positive electrode is made of polymer (PVDF-HFP), plasticizer, and activated material. Then in step 12, a negative electrode is made of PVDF-HFP, plasticizer, and activated material. A separator membrane is then made of PVDF-HFP and plasticizer in step 13. In step 14, the positive electrode, the negative electrode, and the separator membrane are heated under pressure to fuse their PVDF-HFP together for forming a unitary cell. Then, in step 15, a liquid solvent is used to extract the plasticizer from the unitary cell. Finally, the unitary cell is activated by absorbing an electrolyte solution in step 16.
In this manufacturing process, the function of the plasticizer is to facilitate the positive electrode, the negative electrode, and the separator membrane to be formed as films and be processed. The presence of the plasticizer loosens the polymer structure, thereby the unitary cell absorbs more liquid electrolyte, especially after the plasticizer has been extracted from the unitary cell. The extraction process disclosed by this patent utilizes a liquid solvent to separate the plasticizer in the unitary cell. However, the concentration of the plasticizer in the liquid solvent rises to saturation as the extraction time increases, and the extraction efficiency declines therewith; the extraction process becomes complicated and time-consuming because the saturated liquid solvent has to be changed with a new solvent. Moreover, the plasticizer commonly used by the industry is dibutyl phthalate (DBP), which is a toxic material, extra cost is incurred for treating the extracted liquid waste containing DBP, and hence the production cost increases.
Thus, a simple and efficient method for removing plasticizer in a unitary cell, in which the liquid waste containing plasticizer does not need extra treatment, will effectively shorten the process time, enhance the production capacity, and lower the production cost.